Interpretive Summary: Bacteria possess stress management systems that protect against a variety of stressful environments including starvation, thermal stress, acidic pH, osmotic and oxidative stress. Often, the microbial initiation of a protective response against one type of environmental stress can result in the cross-protection against another stress. We identified that microbial stress responses can cross-protect E. coli O157:H7 during extreme acid stress as typically found in the human stomach. Since the pH values of the human stomach can decrease to 1.5-2.5, these acid resistance systems can protect the microbial pathogen during passage through the stomach during infection. This finding may impact food safety, since E. coli O157:H7 may then be able to traverse the stomach to reach the intestine and initiate infection. Industry, university, and government researchers investigating food safety should find this research interesting, as well as the food industry.

Technical Abstract:
Microorganisms may simultaneously encounter multiple stresses in their environment. To investigate the protection that several known Escherichia coli O157:H7 acid resistance systems might provide against both oxidative and acid stress, the addition of diamide, a membrane-permeable thiol-specific oxidizing agent, or hydrogen peroxide were used concurrent with acid challenge at pH 2.5 to determine bacterial survival. The addition of either diamide or hydrogen peroxide decreased bacterial survival in a dose-dependent manner for E. coli O157:H7 during challenge at pH 2.5 following overnight growth in LB MES pH 5.5 (acid resistance system 1, AR1). In contrast, the presence of either glutamate or arginine during challenge provided significant protection against diamide- and hydrogen peroxide-induced oxidative stress during pH 2.5 acid challenge. Oxidative stress protection during acid challenge required gadC and adiA for the glutamate- (AR2) and arginine- (AR3) dependent acid resistance systems, respectively. In addition, maximal protection against oxidative stress in the presence of glutamate required a low external pH (pH 2.5), since pH 5.5 did not protect. This study demonstrates that the glutamate- and arginine-dependent acid resistance systems of E. coli O157:H7 can simultaneously protect against oxidative stress during extreme acid challenge.